The availability of materials used in road pavements, in particular asphalt and aggregate material, has in recent years been reduced. In addition, the materials that are available are becoming more expensive. As a result, there has been an increase in the use of reclaimed asphalt pavement (RAP) in road pavements. RAP is removed and/or reprocessed pavement materials containing asphalt and aggregates. RAP is generated when asphalt pavements are removed, for example during reconstruction or resurfacing of a roadway or to access buried utilities. The removed pavement material is crushed and screened. The resulting RAP product consists of aggregates coated with asphalt cement.
Asphalt pavement is generally removed by milling or full depth removal. In milling, a milling machine removes about 50 mm (2 in.) of pavement material per pass. Full depth removal involves breaking the pavement using a bulldozer or the like. In either case, the removed material may be hauled to a central processing facility, where it is crushed, screened and prepared for use in new pavements. In some cases, the removed pavement material may be pulverized in place.
The RAP may be incorporated into new pavement using typical hot mix or cold mix processes known to those skilled in the art. RAP that is centrally processed may be incorporated into hot mix or cold mix asphalts as an aggregate substitute. Because the RAP includes asphalt cement on the reprocessed material, less asphalt is required in the new pavement. In-place recycling using hot mix or cold mix processes may also be used with RAP. The use of RAP reduces the quantity of new asphalt and aggregate required when paving a road. Details regarding the use of RAP can be found, for example, in Research Results Digest, Number 253, March 2001, “Recommended Use of Reclaimed Asphalt Pavement in the Superpave Mix Design Method: Guidelines”, the contents of which are incorporated herein by reference.
The quantity of RAP that may be included in road pavement material is limited in many countries and in many states in the United States. Typically, regulations limit the amount of RAP that may be included in new pavement to no more than about 25% by weight on average. This limitation has been imposed based upon the stiffening effect of RAP on the final asphalt mix material. The stiffening effect of the RAP is related to some extent to fatigue failures of the asphalt mix which reduces the useful life of the pavement. The stiffening effect of the RAP is the result of aging of the asphalt binder coating the aggregate contained in the RAP. Asphalt aging typically leads to an increase in the stiffness and viscosity of the asphalt binder to the extent that blending of the aged binder with fresh asphalt binder can be non-homogeneous. It would be desirable to increase the amount of RAP that can be used in new pavement materials to reduce the cost and increase the available quantity of materials for pavements.
RAP may also be used in other products that utilize asphalt binders. For example, RAP may be used in asphalt roofing materials or roofing applications, such as in asphalt roofing shingles.